SiC is a semiconductor material, which is superior to Si in view of physical properties and electric properties, although currently predominant material is Si. Specifically, a forbidden band width of SiC is three times wider than that of Si, dielectric breakdown voltage of SiC is seven times larger than that of Si, and thermal conductivity of SiC is three times larger than that of Si. Therefore, SiC is expected to be a semiconductor material for performing a high power and super-low energy loss device for the next generation.
A trench type vertical power MOSFET using SiC is disclosed in, for example, U.S. Pat. No. 6,570,185. A cross sectional construction of this power MOSFET is shown in FIG. 16.
As shown in FIG. 16, in the power MOSFET, a N− type drift layer 102 is formed on a surface of a N+ type SiC substrate 101. A N type region 103 and a P type base region 104 are formed on the N− type drift layer 102 in this order. Further, a N+ type source region 105 is formed on a surface portion of the P type base region 104. Furthermore, a trench 106 is formed to penetrate the N+ type source region 105, the P type base region 104 and the N type region 103 and to reach the N− type drift layer 102. A gate electrode 108 is formed in the trench 106 through a gate oxide film 107. A P+ type layer 109 is formed on a bottom of the trench 106.
In the power MOSFET having the above construction, since the P+ layer 109 is formed on the bottom of the trench 106, current flowing through a channel to be formed in the P type base region 104 flows through the N type region 103 when a voltage is applied to the gate electrode 108. Thus, an on-state resistance of the power MOSFET can be reduced, compared with a case where a device has no N type region 103. This is because the N type region 103 has high impurity concentration, i.e., the N type region 103 has low resistance.
Further, since the P+ type layer 109 is formed on the bottom of the trench 106, electric field concentration is prevented from generating at a corner portion between the bottom of the trench and the sidewall of the trench. Thus, the gate oxide film 107 at that portion is protected from being destroyed.
However, when the P+ type layer 109 is formed on the bottom of the trench 106, it is required to separate a distance between the P+ type layer 109 and the P type base region 104 because of electrical separation therebetween, or it is required to form the N type layer 103 under the P type base region 104, as shown in FIG. 16. Therefore, in the former case, the depth of the trench becomes larger. In the latter case, an additional process is required to form the N type layer 103.